1. Field of the Invention
The present invention relates to an image reader which can feed a document and read document information at a still position from below a glass platen, as well as to an image formation apparatus incorporating such an image reader.
2. Description of the Related Art
A facsimile machine or a digital copier is provided with an image reader which includes an illumination system, an optical read system, a line sensor, and so on. The illumination system illuminates a document and the optical read system focuses a reflected light therefrom on the line sensor to photoelectrically convert image information into signals. There are two ways for such an image reader to read a document. One is a platen type reading in which a document is placed on a glass platen, pressed with a platen cover and read by moving illumination system and optical system. The other is a sheet-through type reading in which a document is automatically fed via an automatic document feeder (ADF) to the glass platen as an exposure position of the optical system and read by fixed illumination system and optical system. The sheet-through type reading can enhance work efficiency of the read operation.
FIG. 17 schematically shows a prior art image reader with three mirrors and a reducing focusing lens. Such an image reader comprises a first scanning member 3 having glass platen 2-2 and a first mirror 3-a, a second scanning member 4 having a second mirror 4-a and a third mirror 4-b, a reducing focusing lens 5, a line sensor 6, an illumination system 7, and an ADF 8. In the platen type reading, a document is placed on the glass platens 2-1, 2-2, and the first scanning member 3 is moved to a read start position 3-2 and the second scanning member 4 is moved to a platen read start position 4-2. A light is reflected by the first mirror 3-a and then by the second and third mirrors 4-a, 4-b, guided to the reducing focusing lens 5, and focused on the line sensor 6. For reading the document, the first scanning member 3 is moved to a read position 3-3 at a predetermined speed, and the second scanning member 4 is moved to a read position 4-3 at half a speed of that of the first scanning member 3.
In the sheet-through type reading, while the illumination system 7, the first and second scanning members 3, 4 are stopped at a position C, a document 1 is fed from a tray A of the ADF 8 to a point B of the glass platen 2-1 at a constant speed. The document 1 is illuminated by the illumination system 7. Then, light from the document 1 is reflected by the first mirror 3-a of the first scanning member 3 then by the second and third mirrors 4-a, 4-b, guided to the reducing focusing lens 5, and focused on the line sensor 6.
Tubular lamps such as xenon lamps, halogen lamps are for use in the image reader as a light source. However, these lamps are disadvantageous since they consume much electric power and generate a lot of heat, leading to increases in power consumption and temperature of the entire unit. For this reason, the tubular lamps have come to be replaced by an LED which consumes less power and generates less heat. However, because the LED is a point light source, it is necessary to dispose an optical system so as to diffuse or gather light for illuminating a document. For example, Japanese Unexamined Patent Application Publication No. 2006-25303 discloses an illumination unit in which LEDs are aligned in an array to illuminate a document via an optical guide. Also, Japanese Unexamined Patent Application Publication No. 2005-278132 discloses an illumination unit in which a long lens system is disposed before arranged LEDs, so as to enhance a degree of light focus in a direction perpendicular to the arrangement of the LEDs. However, this illumination unit still holds problems in increasing the size of the optical system and the number of components, complexing adjustment of the optical system and the illumination system, increasing manufacture costs and the like.
In view of solving the above problems, an illumination unit using an organic electroluminescence (organic EL) has been developed as a surface-emitting light source. Japanese Unexamined Patent Application Publication No. 2000-115470 discloses a technique to illuminate a document with two closely disposed surface light sources and read document information from a space between the two surface light sources. However, light from a light emitting surface of the surface light source tends to be brightest over a center portion thereof despite the fact that a document is read with light from the periphery portions of the light emitting surface. Therefore, this technique has a drawback of low lighting efficiency and impracticality. In order to improve lighting efficiency, Japanese Unexamined Patent Application Publication No. 2007-13913 discloses a technique to increase amount of light at a read position of a document by disposing a reflective member on a surface light source. Further, Japanese Unexamined Patent Application Publication No. Hei 6-217083 discloses a technique to form a large number of microprisms on the light emitting surface of a surface light source in order to enhance directivity of light in an emission direction illuminating a document. Japanese Unexamined Patent Application Publication No. 2006-60528 discloses an illumination unit in which a surface light source is directly disposed at the backside (opposite to a document side) of a glass platen.
Generally, it is assumed that Lambert distribution occurs on a light emitting surface and amount of light illuminating the document decreases inversely proportional to square of a distance between the light emitting surface and the document. Disposing the reflective member between the light emitting surface and the read position increases the distance therebetween; therefore illuminance on the document surface is likely to decrease. It can be configured to collect light on the reflective member. However, it is difficult to adjust the position of the reflective member since illuminance on the document will not be even depending on the position of the reflective member. This further causes an increase of the size of the illumination system. Forming a large number of microprisms on the light emitting surface of the reflective member will increase manufacture costs, and make the distance between the document and the illumination unit longer, decreasing illuminance on the document and lighting efficiency. With regard to disposing the light source at the backside of the glass platen, the illumination unit and the glass platen have to be integrated so that it cannot apply to an image reader with such a structure that the illumination unit moves under the glass platen, as shown in FIG. 10.
Moreover, in recent years there have been demands for reducing the size of an optical system of the image reader in a height direction and reducing the thickness of the illumination unit. In order to achieve a decrease in the thickness of the illumination unit, however, the light source needs to be disposed as close to the document as possible (FIGS. 18A, 18B). In this case, an angle θ between emission direction of light and the document is smaller than that when the light and the document distance away from each other (FIGS. 18A). There is a problem that with the angle θ below a certain value, light may not pass through the glass platen towards the air and be totally reflected at the border of the glass platen and the air, disabling the illumination to the document.